Create a key object by reading a private key file. If the private
key is encrypted and C{password} is not C{None}, the given password
will be used to decrypt the key (otherwise L{PasswordRequiredException}
is thrown). Through the magic of python, this factory method will
exist in all subclasses of PKey (such as L{RSAKey} or L{DSSKey}), but
is useless on the abstract PKey class.
@param filename: name of the file to read
@type filename: str
@param password: an optional password to use to decrypt the key file,(more...)